In a dry electrophotographic print engine, a photoreceptive element is initially charged uniformly using known methods such as employing a grid controlled corona charger, or a roller charger. An electrostatic latent image is then formed on the photoreceptive element by image-wise exposing the photoreceptive element using known methods such as a light emitting diode (LED) array, a laser scanner, or an optical exposure system. The electrostatic latent image is then converted into a visible image by bringing the photoreceptive element into close proximity to marking or toner particles contained in a development station and biasing the development station so that the marking particles would be preferentially attracted to the latent-image bearing portions of the photoreceptive element and repelled by the portions of the photoreceptive element that do not bear latent image information. The toner is then transferred to a receiver such as paper, generally by pressing the paper into contact with the toned photoreceptive element while exerting an electrostatic field to urge the toner to the receiver. Alternatively, the toner can first be transferred to a transfer intermediate member and then from the intermediate member to the receiver.
Color images are made by making electrostatic latent images corresponding to the subtractive primary colors, cyan, magenta, yellow, and black, converting the electrostatic latent images into color images corresponding to those subtractive colors, and transferring the images, in register, either directly to a receiver or to an intermediate transfer member and then onto a receiver.
The toner or marking particles typically consist of dry particles comprising a polymer binder such as polyester or polystyrene, pigment or other colorant, surface treatment addenda such as nanometer-size clusters of silica, titania, or charge agents. Toner particles typically are between 4 μm and 8 μm in diameter, but may be larger (up to approximately 30 μm in diameter) or between approximately 1 μm and 4 μm. For the purpose of this invention, toner diameter refers to the volume weighted median diameter, as measured with a commercially available device such as a Coulter Multisizer or equivalent. The toner particles typically have a glass transition temperature Tg between approximately 45° C. and 65° C., more typically between 50° C. and 60° C. For the purpose of this invention, toner or marking particles refer to the particles used to transform the electrostatic latent image into a toner image, often referred to as a visible image. The toner particles may contain a colorant such as a pigment or dye. Alternatively, the toner particles can be clear or absent any added colorant.
While monocomponent developers that do not comprise so-called carrier particles are used in dry electrophotographic print engines, it is more common to employ so called two-component developers. In this instance, the toner particles are mixed with magnetic particles, often referred to as carrier particles. The carrier particles are generally larger than the toner particles and are triboelectrically dissimilar to the toner particles so that the toner particles become electrically charged when contacting the carrier particles. The mixture of toner and carrier particles is often referred to as a two-component developer.
Two-component developers are used to transform the electrostatic latent image into a visible image by bringing the charged toner particles into close proximity to the electrostatic latent image bearing photoreceptive element, where the charged toner particles are attracted to the charge pattern making up the electrostatic latent image. The carrier particles are contained and transported by a development station comprising a so-called magnetic brush, as is known in the literature.
After transfer to the receiver, the toner image is fixed to the receiver by fusing. This is generally accomplished by subjecting the toner image bearing receiver to heat and pressure so that the toner is heated to a temperature above its Tg while subjecting the toner image to pressure. This allows the toner to flow and to become permanently fixed to the receiver. In addition, if a color image has been printed, the subtractive primary colored toners flow together to create the full-color print. Application of heat and pressure to the toner image bearing receiver is generally accomplished by passing the receiver between two heated compliant rollers. The durameters of the rollers can vary significantly or be near equal to one another. Load applied between these two compliant rollers results in a fusing nip width that provides the dwell time for melting the toner. As the receiver enters the fusing nip, an increased load to the fuser drive system is created. This can cause the fuser rollers to slow down, thereby slowing the speed of the receiver. This causes the lead edge of the receiver to travel more slowly. If the trail edge of the receiver is driven at a higher speed with a force greater than the beam strength of the receiver, the receiver will buckle. This can cause physical print artifacts or degrade image quality. Moreover, the print engine speed may be altered by the mismatched fuser speed through the receiver coupling between them. These variations can result in nonuniform image gloss, streaks, incomplete fusing, hot or cold offset whereby toner that is either heated too much or too little transfers from the receiver to the fuser roller or color-to color misregistration. These effects can cause print artifacts and can result in damage or increased maintenance to the print engine.